﻿550 DR. T. GROOM AND MR. P. LAKE ON THE [i^OV. I908, 



The compact greenstone (ITo. 1 of the foregoing succession) is the 

 intrusive diabase worked in the Ceiriog Granite Company's quarry- 

 immediately south of the area covered by our map. TTos. 3, 6, & 8 

 are apparently the three ash-bands of the Geological Survey ; 'No. 9 

 is the limestone of Dolhir and Cefngoed, which is correlated by 

 the Survey, as well as by Davies, with the Bala Limestone ; and 

 ISTo. 11 is the upper limestone marked upon the Geological-Survey 

 map. Concerning the lowest band of limestone, jSTo. 5, there 

 appears to be some confusion. In Davies's earlier section along 

 the same line he places this limestone above the second ash (No. 6 

 of his section), and states that it is met with near some old mines 

 in the neighbourhood of Hafod-y-gareg on the Teirw. These old 

 mines still exist, and fossils occur in the beds near by, but we have 

 found no limestone there. The beds above the second ash are, 

 however, often calcareous ; and in Nant Blaen-y-cwm^ there 

 is a small outcrop of definite limestone, at or about this horizon. 

 We have never found any limestone below the ' second ash, and 

 our observations therefore agree with Davies's first section rather 

 than with his last. It may be observed that Sedgwick shows a 

 third band of limestone, besides the two in the upper group, and in 

 his Section YIII places it in the bed of the Teirw ; but the small- 

 ness of the scale employed makes it difficult to identify the several 

 bands of ' porphyry.' 



As for Davies's classification into Llandeilo and Bala, it will 

 appear subsequently that (in the line of his section) there is indeed 

 a considerable palaeontological break at the base of the Dolhir 

 Limestone (JSTo. 9 of his section), and that the fossils above are 

 very different from the fossils below. Nevertheless, both must be 

 referred to the Bala Series, as that group is generally understood, 

 and of the presence of Llandeilo Beds above the lowest ash there 

 is no evidence. 



According to Davies, there is an unconformity at the base 

 of the Llandovery Beds and towards the west they overlap the 

 Hirnant Limestone and the beds below, until they lie almost on 

 the Bala Limestone itself. It may be observed, however, that in 

 his map Davies includes in the Llandovery a large area near 

 Moel Perna which is certainly occupied by Bala Beds ; and even in 

 his line of section the hill of Pentre, the beds of which he calls 

 ' Llandovery,' is in reality formed almost entirely of Bala rocks. We 

 shall show that his identification of the beds close to Glya 

 as Llandovery is correct, but we cannot accept his boundary 

 elsewhere. 



Moreover, in the map accompanying Davies's paper the Taran- 

 nons are restricted to a very short and narrow strip of ground 

 by Moel Perna, and are overlapped on both sides by the Wenlock 

 Beds, while east of Dol-y-Wern the overlap increases until the 

 Wenlock lies directly on the Bala Series. It is difficult to under- 

 stand on what grounds Davies based his boundaries near Moel 



^ Nant Blaen-y-cwm is the direct continuation, west of the limits of our map 

 (PI. LIII), of the valley of the Teirw. The name Teirw is, in this part, applied 

 to a tributary stream that comes in from the north. 



